Midterm review: weeks 1-7

The midterm covers the first seven weeks: how Java is compiled and run, variables and operators, Scanner and String, decision structures, loops, methods and arrays (ArrayList included). This page does not teach anything new; it collects the scattered rules in one place and lists the traps that cost the most points. The questions at the end mix the topics, just like the real exam.

First read the summary table, then solve the questions with pen and paper. For every question you get wrong, go back to its topic and reread the “Common mistakes” table on that page.

One-table summary

The ten most expensive traps

These take the most points. Each one was explained in detail on an earlier page.

Exam tactics

Output (tracing) questions: Draw a trace table in the margin of the paper: a column for each variable, one for the condition, one for what is written to the screen. Run the code line by line. When a variable changes, write the new value below. Tell print and println apart: after println comes a new line. Most mistakes are made in the first round, in the last round and in the value after leaving the loop; check these three separately.

Method writing questions: Partial credit comes in this order:

1. Header: public static <return type> <name>(<type> <param>). If the answer is yes/no, boolean; if a number, int or double; if text, String; if it only prints, void.
2. Initialize the result variable: int sum = 0;, int count = 0;, int max = arr[0];.
3. The loop and the condition inside it.
4. return (if not void), outside the loop.
5. Test on paper with an example.

Even if you have no idea, write steps 1 and 2. A blank paper is zero points.

Error-finding questions: Scan in order: is there a ; at the end of each line, do the curly braces match, is every variable used defined in that block (scope), do the types match (double → int), is the String in double quotes, is the char in single quotes, is it = or == inside an if.

Worked mixed example 1: loop + condition

int total = 0;
for (int i = 1; i <= 4; i++) {
    if (i % 2 == 0) total += i * i;
    else total -= i;
}
System.out.println(total);

i   i % 2 == 0 ?   operation        total
1   F              total -= 1       -1
2   T              total += 4        3
3   F              total -= 3        0
4   T              total += 16      16
5   condition 5 <= 4 F → exit

16

Worked mixed example 2: method + array + pass-by-value

static int f(int[] a, int k) {
    a[k] += 10;
    k++;
    return a[k];
}
// in main:
int[] arr = {5, 1, 4};
int k = 0;
System.out.println(f(arr, k) + " " + k + " " + arr[0]);

1 0 15

Think about three things separately:

• a[0] += 10: because the array came by reference, arr[0] in main became 15.
• k++: the method’s own copy of k became 1. The k in main stayed 0.
• return a[1]: 1.

Three different rules work on the same line; a midterm question looks exactly like this.

Question type → which page to go back to

If you got a question wrong, find which rule you missed and go back to that page. The table below ties most questions to a single rule:

Last-night checklist

• Can you write the three Scanner lines (import, new, nextInt) from memory?
• Can you explain the difference between 1 + 2 + "A" and "A" + 1 + 2?
• Can you write the left-aligned, inverted and right-aligned star triangles with their loop bounds?
• Can you write the methods isPrime, sumDigits, max, countDivisors, getSeason without looking?
• Can you write methods that find the sum, the average (double!) and the maximum of an array?
• Do you know the difference between remove(1) and remove(Integer.valueOf(1))?

If you say yes to all of them, solve the questions below and note down your mistakes.

Common mistakes

System.out.println(5 + 7 / 2 * 2 + "x" + 5 % 3);

int a = 3;
int b = a++ * 2 + --a;
System.out.println(a + " " + b);

char c = 'b';
int x = 0;
switch (c) {
    case 'a': x += 1;
    case 'b': x += 2;
    case 'c': x += 3; break;
    default: x += 10;
}
System.out.println(x);

int pairs = 0;
for (int i = 1; i <= 4; i++)
    for (int j = i + 1; j <= 4; j++)
        if ((i + j) % 2 == 1) pairs++;
System.out.println(pairs);

int n = 3, s = 0;
do {
    n--;
    if (n == 1) continue;
    s += n;
} while (n > 0);
System.out.println(n + " " + s);

static int f(int[] a, int k) {
    a[k] += 10;
    k++;
    return a[k];
}
// in main:
int[] arr = {2, 4, 6, 8};
int idx = 1;
int r = f(arr, idx);
System.out.println(r + " " + idx + " " + arr[1]);

String w = "Programming";
int cnt = 0;
for (int i = 0; i < w.length(); i++) {
    if (w.charAt(i) == 'm' || w.charAt(i) == 'g') cnt++;
}
System.out.println(cnt + " " + w.indexOf('g') + " " + w.substring(3, 6));

ArrayList<Integer> list = new ArrayList<>();
for (int i = 0; i < 4; i++) list.add(i * i);
list.add(1, 7);
list.remove(Integer.valueOf(4));
System.out.println(list + " " + list.get(2));

static String t(int x)    { return "I" + x; }
static String t(double x) { return "D" + x; }
// in main:
System.out.println(t('A'));
System.out.println(t(5L));
System.out.println(t(9 / 2));

double avg = 0;
int[] m = {7, 8, 10};
for (int v : m) avg += v;
avg /= m.length;
System.out.printf("%.2f %d%n", avg, (int) avg);

int total = 0;
for (int i = 0; i < 3; i++) {
    int sq = i * i;
    total += sq;
}
double avg = total / 3;
String msg = "avg";
if (msg == "avg") System.out.println(sq);